In order to produce concrete products of good quality and high strength essential for building blocks, the curing cycle for the concrete is critical. Thus, the temperature, carbon dioxide and humidity must be controlled at various times during the curing cycle. For example, Simunic in U.S. Pat. No. 3,492,385, Jan. 27, 1970 proposes a conveyor system for passing the products through alternating hot and cold regions, while varying hot air temperature and carbon dioxide content and introducing humidity with sprayed water during the curing cycle.
This system however imposes considerable disadvantages, in part because of complex and expensive kiln construction having a limited life because of the necessity to operate a moving conveyor system through the corrosive kiln atmosphere. Also, the energy expenditure is high because of the cyclic requirement to cool hot products and heat cooled products. Quality of products such as building blocks formed of dry mixture would be difficult to maintain, because of the difficulty in preventing cracks or breakage of products during exothermic or water exhaustion phases of the curing cycle. Also, green concrete, particularly fragile dry-mix concrete for building blocks, can be damaged in moving during the green phase. Controls of the curing cycle in this system cannot be controlled critically enough to reduce energy, or to increase quality by reduction of breakage or weakening during the curing process, for example because exothermic curing reaction causes excessive temperatures and dry heating air isolated from the humidifying regions may cause excessive water loss and thus fissures or cracks curing critical cure phases.
Thus, it is an objective of this invention to produce more efficient, longer life kilns needing little maintenance and better control of curing cycle conditions, thereby leading to improved quality products produced at lower cost.
Similarly concrete products are conveyed through a kiln in Murray, U.S. Pat. No. 4,427,610, Jan. 14, 1984, inconsistent with the fragile status of green concrete building blocks. In this system the products are cured by air of controlled temperature and CO.sub.2 content. This kiln and curing method however is subject to the same criticism regarding kiln life and maintenance, and further fails to control the humidity level, critical to quality and strength by the prevention of excessive water losses from the product pores causing fissures and cracking.
Humidity is maintained greater than 90% throughout the curing cycle by Wauhop, Jr., U.S. Pat. No. 4,099,337, Jul. 11, 1978. Thus a water bath in the kiln by evaporation and diffusion through the kiln as aided by water sprayed on the pool surface, provides a substantially constant kiln humidity and temperature through which the products on pallets are moved and discharged after about six hours of curing. This system requires a long curing time and moves the fragile products at all cure stages through the kiln, and thus introduces the aforementioned problems of product strength, kiln life and kiln maintenance. Furthermore, this system is deficient in being able to control the atmosphere inside the kiln in a manner consistent with an optimized curing cycle.
It is therefore a general objective of this invention to improve the state of the concrete curing art by introducing improved kilns and curing methods. Other objects, features and advantages of the invention will be recognized from the following description, drawings and claims.